Free space optical communication has attracted considerable attention recently for a variety of applications. Atmospheric turbulence can degrade the performance of free-space optical links (e.g., tens of decibels), particularly over ranges of the order of 1 km or longer. Lack of homogeneity in the temperature and pressure of the atmosphere, for example, can lead to variations of the refractive index along the optical transmission path. Such refractive index variations can deteriorate the quality of the received image and can cause fluctuations in both the intensity and the phase of the received optical signal. These fluctuations, which are also referred to as fading, can lead to an increase in the link error probability, limiting the performance of optical communication systems.
For optical communications systems design, the effects of fading should be accounted for to ensure reliable system operation. In suitable atmospheric conditions, received optical signals may be strong which could saturate a photodector at the optical receiver. In poor atmospheric conditions, weak optical signals need to be boosted over background noise levels in order to properly distinguish transmitted data from noise. Thus, optical systems needs to be designed to operate over a fairly large dynamic power range accounting for strong signal strength under optimal conditions and weak signal strength under poor conditions.